Converted app buttons to used the Polymer template
Revert "Converted app buttons to used the Polymer template"

This reverts commit b30b30c7be9bb9f78b39886b1641e1d689afc918.
Fixed: double-execution of webkitStartDart() broke polymer-element instantiations.

webkitStartDart() is already invoked by polymer/boot.js, which spark.html includes.